It is a convenient practice to physically separate a digital computer from the keyboard and monitor by which the computer is monitored and controlled. The monitor and keyboard may be in the form of a single housed unit or, as is more common today, the monitor and keyboard are separate. One of the most popular microcomputers in use today is the IBM personal computer family and its clones made by others, which generally utilize the same or a similar arrangement for interconnecting a keyboard and monitor to the computer.
As a matter of convenience, there are separate electrical connectors on these computers for mating plugs, one plug connecting to a monitor cable and the other to a keyboard cable. The opposite ends of these cables are provided with plugs which plug into a monitor connector of a monitor and a keyboard connector of a keyboard. Normally, these cables are on the order of three to six feet, enabling some, but limited, separation from the computer. This typically requires that the computer and work station be located together.
It is to be appreciated that there are situations, in fact, many, where it is desirable to separate the computer from the work station. This may be due to space considerations or an inhospitable environment for a computer.
Separation has been attempted by extending the length of the separate connecting cables described above, but it has been found that where separation is generally greater than 12-18 feet, reliability of keyboard operation and quality of the video display is sacrificed. If a single cable housing conductors conveying both video and keyboard signals is attempted, as would be the ideal approach, signal interaction becomes intolerable due to induced crosstalk between keyboard and video signal conductors. Although it is believed others have attempted to solve these problems, which have been present for years, applicants are unaware of any successful single-ended driven TTL cable system has been accomplished other than by applicants.
Applicants have devised trouble-free, single ended, TTL driven computer/monitor extension systems which have been proven to extend digital color and monochrome video and keyboard communications between a computer and keyboard and monitor up to 150 feet. These systems are subject of U.S. Pat. No. 4,885,718, issued Dec. 5, 1989, to applicants, which is a parent case of co-pending application Set. Nos. 07/447,010, filed Dec. 5, 1989, and 07/488,710, filed Mar. 10, 1990. Of these, it is believed that application Ser. No. 07/447,010 is the closest to one embodiment of the present invention. However, in all of these systems of the prior art, even with special shielding techniques, when it was attempted to extend digital color video and keyboard communications past approximately 175 feet, signal interaction again became intolerable.
Application Ser. No. 07/447,010 utilizes TTL Schmitt-triggered amplifiers in a first signal conditioning circuit to amplify video signals from a computer and provides these amplified signals via a series resistor to an extension cable. At the other end of the cable, Schmitt-triggered TTL amplifiers amplify the degraded video signals prior to inputting them to a monitor. Significantly, this circuit is limited to reliable extended TTL transmissions of color video signals up to approximately 150 feet because of signal degradation. Applicants extend this range to a maximum of approximately 300 feet by using monochrome monitors.
In application Ser. No. 07/488,710, analog video communications are extended to approximately 300 feet by utilizing a discrete current amplifier near the computer for the R, G, and B analog video signals to modulate signal current flow through the 300-foot cable. A second current amplifier proximate the monitor boosts the signal prior to inputting them to the monitor. Significantly, the horizontal sync pulse, in an analog situation, was prone to cause the interfering crosstalk. Accordingly, to reduce this crosstalk, the HS signal was converted to a limited-amplitude signal which is further attenuated by the 300-foot cable in order to reduce noise.
Accordingly, it is the object of the present invention to provide TTL driven, sing/e-ended extension communications systems among a computer, monitor, and keyboard which allow extensions of up to 250 feet, 400 feet, and 1,000 feet for the embodiments described herein.